The DA 42 does seem to offer great performance for the money. Assuming 165 kts, 12 gph and $5 / gal for JET-A, the fuel costs about 36c per mile.
My assumptions for a C172 are 115 kts, 9 gph and $6 / gal for 100LL giving a fuel cost of 47c per mile.
Of course, this gets more than eaten up by the costs of maintaining a second engine.
As I read somewhere recently, flying a twin means that you are twice as likely to experience an engine failure than in a single but only 1/4 as likely to experience a total loss of power.
I am not sure to how to assess the relative risk of a single with a chute, a few bottles of water etc and a twin with all of the problems around Vmc for a low hours pilot flying over the desert and mountains.
The next step for me is to try to get a MEL this spring and then fly all of the options before making up my mind.